Possibly the points, if that is what the bike has.....have closed up some.
Check ALL electrical connections......clean and grease.
Could be CARB troubles....Perhaps you adjusted them a little TOO much....Maybe BACK off the settings that you changed, BACK to where they were before.
New or clean and gap the SPARK PLUGS.
Backfiring is usually TIMING and or LEAN condition.
Good Luck and please post back.
1973 /5 LWB Toaster
spo123 screwed with this post 11-16-2012 at 12:26 AM